(1) Field of the Invention
The present invention relates to the field of sensors for measuring the properties of a sheet, and more particularly to a sensor and system including a caliper gauge for measuring sheet compressibility subjected to a plurality of pressures.
The invention uses the compressibility measurements to compute a compression modulus of elasticity which can be used to derive various Z-directional properties of the sheet, such as the tensile modulus of elasticity, the Z-directional tensile strength, extensional stiffness and Scott bonding.
(2) Description of the Related Art
Various types of caliper gauges are known in sensor technology and are used for measuring the thickness of rapidly moving sheet material. One type of caliper gauge is called a "contacting caliper gauge." Contacting caliper gauges typically have two opposing pads which are forced into contact with opposite sides of a sheet. The distance between the pads is measured and directly relates to the sheet thickness or "caliper."
It has been recognized that the aerodynamic design of the caliper pads must be considered if the pads are to be maintained on or near the sheet surface with relatively little external force. A previous caliper gauge having aerodynamically designed caliper pads is disclosed in the commonly assigned U.S. Pat. No. 4,901,445 to Mathew G. Boissevain, et al. This patent is incorporated herein by reference.
However, applicants are not aware of caliper gauges being used to simultaneously measure the compressibility of a sheet at a plurality of pressures or using such measurements to compute a compression modulus of elasticity which can be used to compute the tensile modulus of elasticity and to derive various Z-directional physical properties of the sheet, such as Z-directional tensile strength, extensional stiffness and Scott bonding.
One of the most critical properties involved in the manufacture of paper is its strength. Virtually all paper manufactured is sold with a strength specification of some sort, and acceptance of a manufacturer's paper depends on being able to meet this requirement. Paper strength has three basic orientations: 1) the machine direction, 2) the cross-direction, and 3) the Z-direction. The machine direction refers to the primary direction of sheet travel through the papermaking machinery. The cross-direction refers to the direction across the width of the sheet, in the plane of the sheet and perpendicular to the machine direction. The Z-direction extends perpendicular to the plane defined by the machine and cross-direction and is also referred to as the thickness direction.
A previous system for continuous determination of sheet strength in the cross-direction and machine direction is disclosed in the commonly assigned U.S. Pat. No. 4,866,984 to Paul J. Houghton which is incorporated by reference. This system does not, however, determine sheet strength in the Z-direction, and therefore does not provide a complete picture of paper strength. In addition, due to the alignment of wood fibers, which are the main constituent of paper, the strength for each orientation is substantially different.
The Z-directional strength of paper is usually given in terms of an empirical, destructive test. A common test is the Scott bond test where the Z-directional strength is determined by measuring the bonding between the different layers of fibers through the sheet. In the Scott bond test, a strip of paper or sample is delaminated by applying an in-plane shear force. One side of the sample is double-taped to a fixed support. The other side is taped to an "L" bracket. A pendulum is then released to hit the vertical side of the bracket and shear the sample. The energy lost in the delamination can be measured from the stopping position of the pendulum.
In another such destructive test, the Z-directional tensile test, a sample is delaminated by pulling on both surfaces in the Z-direction with an equal force. Double-sided tape is used on each side to transfer the stress to the sample. The paper strength is determined by measuring the force exerted on the paper when it ruptures.
Of course, neither test can be performed "on-line" as the paper is being manufactured in order to avoid the production of substandard material. Instead, the sample must be taken from the end of a reel after the reel has been completed. Since papermaking is a high speed continuous process, large amounts of paper can be easily produced before strength can be confirmed by a later measurement. The result of the time-consuming, off-line testing is that the strength measurement provided by perhaps several square feet must be accepted as representative of the thousands of square feet making up the reel. Consequently, it would be highly desirable to measure paper strength on-line while it is being manufactured.
An article by Hall, On-Line ultrasonic Measurement of Paper Strength, Sensors (1990), based on research at the Institute of Paper Science and Technology, describes the use of commercial fluid filled wheels which are adapted for out-of-plane ultrasound velocity measurements and caliper measurements for moving paper webs. However, no on-line data is presented and there is no apparent disclosure of how to correlate these measurements with Z-directional paper strength.
Nonetheless, Z-directional strength is important to monitor and control on-line, both to the efficiency of the paper mill and the efficiency of the processes in the converting operations. In a paper mill, there are several processes, such as those employing sizing presses and coaters, where highly viscous materials are applied to the paper. In the process of applying and drying these materials, the sheet or portions of the sheet can be pulled apart in the Z-direction, causing build-up on the applicators and dryers. In addition, there are other areas where either the surface of a calender roll or the surface of the sheet is dampened which causes the sheet to adhere to the roll.
However, too high a value of Z-directional strength can also cause problems. For example, if the Z-directional strength is obtained from too much densification of the sheet by wet pressing, or other means, then certain properties such as opacity, folding stiffness and tear strength will be reduced. Also, it can be generally stated that the higher the strength, the higher the cost of manufacturing the paper.
The company that buys the paper and converts it into a product has similar types of Z-directional strength requirements. Many of the converting operations, such as corrugators, printers, coaters and laminators, also apply substances of high viscosity that create a Z-directional force on the sheet of paper. Here again, if the Z-directional strength in the paper is too low, portions of the sheet will be pulled apart or delaminate in the Z-direction causing either sheet breaks or build-up on the applicator rolls or drying equipment.
Thus, it would be desirable to be able to obtain the Z-directional strength, as well as other Z-directional properties such as tensile strength, extensional stiffness and Scott bonding on-line to achieve the strength required by both the papermaker and the converter, while at the same time optimizing cost and manufacturing efficiency.